1. Field of the Invention
The present invention relates to a packet transmission device and control method thereof, and in particular to a packet transmission device for dividing packet data inputted into predetermined length user data to be outputted and a control method thereof.
2. Description of the Related Art
Prior art examples [1] and [2] of a packet transmission technology as mentioned above will now be described referring to FIGS. 16-18.
A radio communication system 1 shown in FIG. 16 indicates an arrangement of a system to which the prior art examples [1] and [2] are applied and is composed of a core network 10, two radio network control devices 20_1 and 20_2 (hereinafter, occasionally represented by a reference numeral 20) connected to the core network 10, four base stations 30_1-30_4 (hereinafter, occasionally represented by a reference numeral 30) managed by the radio network control device 20, and a mobile equipment 40 connecting a call to the radio network control device 20_1 through the base station 30_1.
A downlink packet transmission (core network 10 mobile equipment 40) in the radio communication system 1 is performed, as shown by the solid lines in FIG. 16, by the radio network control device 20_1 dividing packet data SDU (Service Data Unit) received from the core network 10 into user data PDU (Protocol Data Unit), with a size prescribed by an RLC (Radio Link Control) protocol, to be transmitted to the mobile equipment 40 through the base station 30.
Also, an uplink packet transmission (mobile equipment 40→core network 10) is performed, as shown by the dotted lines in FIG. 16, by the radio network control device 20_1 assembling the user data PDU received from the mobile equipment 40 through the base station 30 into the packet data SDU to be transmitted to the core network 10 (see e.g. patent document 1).
Recently, an introduction of an HSDPA (High Speed Downlink Packet Access) system which enables a high speed downlink packet transmission has been considered as a packet transmission standard in such a radio communication system 1. The HSDPA system, where the packet transmission is performed by using a common channel called an HS-DSCH (High Speed Downlink Shared Channel), realizes communications at a maximum transmission rate “14.4 Mbps” exceeding a maximum transmission rate “2 Mbps” of the existing packet transmission standard.
Prior Art Example [1]: FIG. 17
FIG. 17 is a diagram where an arrangement of the radio network control device 20 is emphatically shown within the radio communication system 1 shown in FIG. 16.
The radio network control device 20 is composed of a core network interface 100 connected to the core network 10 through “i” units of channels CH1-CHi (hereinafter, occasionally represented by a reference character CH), a packet data transfer function 200 transferring the packet data SDU received through the core network 100 to the inside of the device 20 itself, a user data converter 300 dividing the packet data SDU transferred from the packet data transfer function 200 into predetermine length user data PDU to be outputted, an HSDPA frame converter 400 converting the user data PDU outputted from the user data converter 300 into a frame (hereinafter, referred to as HSDPA frame) FR with a format prescribed by the HSDPA system to be outputted by using the HS-DSCH, and a base station interface 500 transmitting the HSDPA frame outputted from the HSDPA frame converter 400 to the mobile equipment 40 through the base station 30.
Also, the user data converter 300 has low speed user data-conversion cards 310_1-310_i (hereinafter, occasionally represented by a reference numeral 310) provided corresponding to each of the channels CH1-CHi. It is to be noted that the low speed user data-conversion card 310 is generally used in the radio communication system 1 to which the above-mentioned existing packet transmission standard is applied, and has a resource amount (e.g. a capacity or the like of a memory (not shown) for temporarily storing the packet data SDU) in order to process the packet data SDU transmitted by a sufficient data amount within the maximum transmission rate “2 Mbps” of the packet transmission standard.
In operation, as shown in FIG. 17, when 3 pieces of packet data SDU1-SDU3 are inputted through the channel CH1 for example, the packet transfer function 200 transfers the packet data SDU1-SDU3 to the low speed user data-conversion card 310_1 provided corresponding to the channel CH1.
The low speed user data-conversion card 310_1 having received the packet data SDU1-SDU3 divides the packet data SDU1-SDU3 respectively into the predetermined length user data PDU1_1-PDU1_k, PDU2_1-PDU2_1, and PDU3_1-PDU3_m to be provided to the HSDPA frame converter 400.
The HSDPA frame converter 400 generates, upon every receipt of the user data PDU, the HSDPA frame FR having a predetermined header HD_FR added to a predetermined number of the user data PDUs, and transmits it to the mobile equipment 40 through the base station interface 500.
In such a prior art example [1], the processing of the low speed user data-conversion card 310 is congested with a speed-up of the transmission rate due to the introduction of the HSDPA system, so that a transmission delay of the user data PDU may occur.
In order to address this problem, a prior art example [2] which will be described hereinafter has already been proposed.
Prior Art Example [2]: FIG. 18
FIG. 18 shows a radio network control device 20 where the low speed user data-conversion cards 310_1-310_i shown in FIG. 17 are respectively replaced with high speed user data-conversion cards 320_1-320_i (hereinafter, occasionally represented by a reference numeral 320) which can output the user data PDU at the maximum transmission rate prescribed by the HSDPA system.
In this case, the high speed user data-conversion card 320 can output the user data PDU without an occurrence of the transmission delay, thereby enabling the radio network control device 20 to fast transmit the HSDPA frame FR to the mobile equipment 40.
[Patent document 1] Japanese Patent Application Laid-open No. 2004-194064
In the above-mentioned prior art example [2], it is possible to enhance a speed of the transmission rate of a packet by newly using (adding) a high speed user data-conversion card. However, since such a high-performance conversion card is expensive, there has been a problem of development costs being increased. Also, it is preferable to use such a high speed user data-conversion card as few as possible in terms of an effective use of the existing resource such as the existing low speed user data-conversion card.